Currently, defects in wafers can be detected by comparing a target die on the wafer to reference dies on the wafer. Inspection systems accomplish this by taking images of the target and reference dies for comparison purposes. In particular, detecting the defects often involves performing two separate comparisons to generate two separate results, one comparison being between the target die and one of the reference dies and another comparison being between the target die and the other one of the reference dies. Any similarity between the two separate comparison results is generally used as an indicator of a defect in the die. Detected defects are then used as the basis for qualifying the design of the wafer (e.g. determining areas of the design that are systematically prone to defects), and further for characterizing the process window used by a manufacturing tool to fabricate the wafer (e.g. the parameters in which the manufacturing tool fabricates the wafer within the specifications of the design).
Prior art FIG. 1 shows traditional layout for a wafer having a plurality of target dies in a column 102, each being a same pattern modulated (i.e. amplified) by a different combination of parameter (e.g. focus (F) and exposure (E)) values, and further having a plurality of reference dies in columns 104, 106 situated on either side of the column of target dies and each being a nominal (i.e. not modulated) version of the same pattern. Thus, for any particular one of the target dies in column 102, a reference die from column 104 and a reference die from column 106 may be used for detecting defects in the particular target die (see box 108). While the reference dies are shown as being adjacent to the target die, this is not necessarily always the case. For example, in other wafer configurations the reference dies for any particular target die may be those closest, but not necessarily adjacent, to the particular target die.
Unfortunately, traditional methods related to the above described defect detection involve techniques that are inefficient and thus time and resource intensive. Just by way of example, current methods for process window characterization (e.g. process window qualification (PWQ) methods) require numerous scan iterations, including a separate test for each die which further involves a separate scan for each modulation of the die.
To compound the limitations mentioned above, current methods further involve estimating the inspection sensitivities separately for each modulation. This may be accomplished using an Initial Threshold Finder (ITF), but in any case requires a preliminary scan of every modulated die to assess defectivity of the modulated die and assign sensitivity to the modulated die, so that the following inspection scan of the modulated dies is performed based on the assigned sensitivities
There is thus a need for addressing these and/or other issues associated with the prior art.